The present invention relates to a heat transfer ink ribbon which forms images on a printing medium upon heating by a thermal head or laser beam in response to signals. More particularly, the present invention relates to an improvement on a binder resin contained in the ink layer.
One of the heat transfer recording methods is the sublimation transfer recording method. This method employs an ink ribbon consisting of a heat-resistant substrate and an ink layer formed thereon which contains a sublimation dye. At the time of printing, the ink ribbon is placed on a printing medium such that the ink layer comes into close contact with the dye-accepting surface of the printing medium which is formed from a polyester resin. Printing is effected by heating the ink ribbon from the opposite side of the ink layer by means of a thermal head which produces a heating pattern in response to an image pattern to be transferred. Upon heating, the ink ribbon permits the sublimation dye to be transferred to the printing medium through sublimation. In this way a desired image is formed on the printing medium.
What is important for this kind of ink ribbon is gamma (.gamma.), which is defined as the tangent of the slope of the straight line part of the characteristic curve obtained by plotting the amount of energy applied (on the abscissa) against the reflection density of transferred ink (on the ordinate). Gamma determines the properties of an ink ribbon. A high gamma value is desirable if printing with a high density is to be performed in a shorter time. (The currently available ink ribbon takes 60-90 seconds for printing.) On the other hand, a high gamma value is undesirable where an image needs gradation. With a high gamma value, it is difficult to reproduce the gradation of a photograph which usually has a density in the range of 0.3 to 0.8. The poor reproducibility is due partly to heat accumulation in the thermal head and partly to fluctuation in the heating time. Several attempts have been made to eliminate these drawbacks by changing the ratio of the dye to the binder resin or by changing the kind of the binder resin.
Changing the ratio of the dye to the binder resin has a good effect on the high-density part but has a very little effect on the low-density part. For the ink ribbon to reproduce the gradation at an adequate printing speed, it is desirable that the gamma value be small for the low-density part and large for the high-density part. This is not achieved by the above-mentioned remedy.
The conventional binder resin used for the ink ribbon includes cellulose, polyvinyl butyral, polyvinyl acetal resins (e.g., polyvinyl acetoacetal), and vinyl chloride resins. These resins do not reproduce gradation satisfactorily because they vary in gamma depending on the amount of energy applied.